This invention relates to electrical fuses and more specifically relates to a resettable fuse implemented by solid state MOSFETs.
Electrical fuses, for example, those for automotive use, are metal fusible links which carry the current in the circuit being protected. These fuse links melt or vaporize at a given current level to open the circuit being protected. They usefully provide an open gap in the circuit to protect personnel who work on the circuit from voltages which might otherwise appear across the open circuited load. The fuses must be replaced after their operation. Circuit breakers may also be used to protect electrical circuits, but they are large and expensive.
Solid state fuses are also known in which transistors and thyristors are placed in series with the load and turn off in response to a load fault condition. These devices however, do not provide the sure protection of the open gap of a fuse or contact air gap of a circuit breaker.
It would be desirable to provide a solid state fuse, that is, one employing solid state controllably conductive devices such as, MOSFETs, IGBTs, thyrsitors and the like which would also provide certainty that an apparently opened circuit load can be worked on without requiring an added air gap type device.
In accordance with the invention, a load is protected by a pair of controllably conductive devices, one in series with the load and the other in parallel with the load. The series device is normally conductive and is turned of in response to a predetermined fault condition. The parallel device is normally off, but can be turned on, with a short delay after the turn off of the series device, in response to a fault if a potential appears across the load. Thus, after a fault and the xe2x80x9copeningxe2x80x9d of the fuse, a positive short is applied across the load to permit the safe maintenance of the load.
In a preferred embodiment of the invention the series and shunt devices are implemented as power MOSFETs. The shunt device may be a normally conductive depletion type MOSFET which is turned off by the application of a gate signal for normal operation. An LED indicator may be connected across the series device to indicate the turn off of the device, and thus the operation of the xe2x80x9cfusexe2x80x9d or the opening of this circuit.
An IC control may be provided along with a current sense circuit to provide signals to turn off the series device in response to the fault, and to turn on the shunt device, (or enable it for conduction if a voltage is applied across the load after turn off of the series device) preferably with a short time delay after the turn off of the series device.